1) Field of the Invention
The present invention relates to an anti-doming composition for a shadow-mask particularly to 1) an anti-doming composition including a mixture of electron reflecting material such as bismuth oxide, and a zeolite to restrict temperature increase by coating the composition on the shadow-mask for a CRT (cathode-ray-tube), 2) an anti-doming composition including lead (Pb) and at least on of ZnO, B.sub.2 O.sub.3 and Bi.sub.2 O.sub.3 to protect mechanical expansion of a shadow-mask and to restrict temperature increase by coating the composition on the shadow-mask for a CRT 3) an anti-doming composition including ferroelectrics such as PZT (PbZrTiO.sub.3), PT (PbTiO.sub.3), PZ (PbZrO.sub.3) or PLZT [(PbLa)(ZrTi)O.sub.3 ] to restrict temperature increase by coating the composition on the shadow-mask for a CRT to change the energy of electron beams into non-thermal energy, and a process for preparing the same.
2) Description of the Related Art
In a conventional shadow-mask-type CRT, graphic images are reproduced by red, green, and blue electron beams emitted from means for producing them. The electron beams pass through a hole of a shadow-mask, converge into a point, and collide with red, green, and blue phosphors formed on a phosphor screen of an inner surface of a panel.
The shadow-mask used in the color CRT has a role of selecting an electron beams for a particular color among the electron beams emitted from an electron gun. The shadow-mask is generally produced with an AK(aluminum killed) steel which has a hundred thousand and tens of thousands hole formed by photolithography.
A general process for preparing the shadow-mask is more particularly described hereafter.
To allow a flatness and setting property to a mask panel, an AK steel is subjected to a roller leveling treatment causing a plastic deformation. Thereafter, the process for preparing holes in the mask panel is performed by photolithography. The photolithography process is performed with the sequence of coating a photoresist, exposing, developoing and etching the panel. According to the etching process, the panel has a number of holes passing electron beams through. The panel having holes is heat treated in a high temperature and hydrogen gas to give a ductility (annealing process) and then a forming process is performed to change the panel into a certain form by a press. After the forming process, the panel is subjected to a degreasing process to remove impurity attached on the surface of the panel. Thereafter, a blacking process is performed to improve anti-doming property of the shadow-mask.
About 20% of electron beams emitted from an electron gun and arrived in shadow-mask pass through the shadow-mask and cause to a luminescence of a phosphor screen. And, about 80% of the remaining electron beams are absorbed in the shadow-mask and cause to thermal expansion of the shadow-mask. In this circumstance, a temperature of the shadow-mask increases to about 80 to 90.degree. C. As mentioned above, we call the thermal expansion of a shadow-mask as a doming phenomena. The doming phenomena results movement of holes in the shadow-mask, which induce change of luminescence positions, further more causes luminescence of different colors. Therefore, the doming phenomena decreases a color purity of CRTs. According to the requirement of preventing the doming phenomena, various kinds of anti-doming methods have developed.
U.S. Pat. Nos. 4,665,338 and 4,528,246 disclosure processes for preparing CRTs using a shadow-mask which is produced with an Invar steel to prevent decrease of color purity according to the doming phenomena. The Invar steel shows a preferable anti-doming property because it has 1/10 coefficient of expansion of an AK steel, 11.7.times.10.sup.-6 /K. Therefore, the Invar steel is usually used as a material of a shadow-mask for above 15 inch CRT. However, the Invar steel has problems of high cost and difficulty during processes.
Korean patent application No. 86-1589 disclosures and electron reflecting layer coated on the one side of an electron gun with heavy metals such as lead (Pb), bismuth (Bi) and tungsten (W) by aqueous emulsion method to reduce the heating caused by the electron beams. However, the metal layer cannot be easily coated, and anti-doming ratio is only about 30%, and the disclosed process has a problem of adjusting to a practical mass production. Additionally, the tungsten layer is usually oxidized at above 300.degree. C. Therefore, the layer has a problem of vigorous oxidation at the temperature of about 450.degree. C. reached in the process of baking and sealing.